224 



THE MECHANICAL FUNCTIONS. 



the section (b b), which lies on each side of the 

 proper apex of the shell, and which forms the ap- 



117 



parent base. The lines on this part of the section 

 indicate the thickness which each successive whorl 

 had originally, and when it was itself the outer- 

 most whorl. The section also shows the vitreous 

 deposit which lines the upper parts of the cavity, 

 and which completely fills up the smaller turns of 

 the spire, near the apex.* 



There are, indeed, instances among shells of the 

 total removal of the interior whorls. This is found to 

 occur in that of the genus Auricula, which are mol- 

 luscous animals, respiring by means of pulmonary 

 organs. In the young shell of this tribe, the parti- 

 tions which separate the cavities of the whorls are 

 incomplete, and twine parallel to each other; but 

 they wholly disappear as the animal approaches to 

 maturity. In other cases, the animal is found to 

 remove exterior portions of shell formerly deposited, 

 when they lie in the way of its further growth, and 



* Fie;. 117, which is a transverse section of the same shell, shows 

 the spiral convolutions, and the comparative thinness of the inner 

 portions. It also forms a striking contrast with a similar section of 

 the shell of the Cypreea, Fig. 114; p. 221. 



